Many types of hoisting devices, such as gantry cranes, bridge cranes, overhead gantries and the like, employ large horizontal beams supported over the lifting area. The beams are normally movable longitudinally on fixed rails, tracks or on guidable wheels which permit the gantries to be moved along the open ground. The beams are elevated above the lifting area by raised tracks or rails, or by the provision of elongated supports, or legs, at each end of the gantry structures. The hoisting operation is accomplished by a winch or similar device which raises and lowers cables attached to a lifting frame, hook, or other load-engaging means from which the load is suspended. Usually, the load-carrying cables are attached to one or more trolleys which run transversely along the gantry beam. This arrangement permits the load to be moved vertically by the hoisting apparatus, transversely by the trolleys, and longitudinally, by moving the beam.
Because the load is suspended by flexible cables, longitudinal or transverse movement of the hoisting apparatus tends to cause sway of the load being carried, which sway is considered undesirable. This is particularly true where the gantry crane is being used to handle large cargo containers or truck trailers in dockyards or railyards. The loads in such instances require accurate placement and any sway induced during movement of the load must be permitted to decay before final placement of the container, etc. This obviously is time consuming and expensive.
In the past, efforts have been made to reduce the sway of loads suspended from such hoisting apparatus. Such efforts have included reeving arrangements in which the hoisting cables diverge outwardly and upwardly and thus tend to resist sway. See, for example, U.S. Pat. Nos. 3,825,128 and 3,086,661. Other patents have suggested the use of a separate stabilizing system (Tacks, et al, U.S. Pat. No. 3,746,182) and the use of tag lines (Crittenden, U.S. Pat. No. 3,532,324). Another solution has been to provide a horizontal stabilizing beam which is guided on the gantry legs and raised and lowered along with the load to provide a stabilizing support. Such structures are shown in U.S. Pat. Nos. 3,161,309; 3,176,853; and 3,251,496.
Co-pending patent application Ser. No. 731,585 of Harris, entitled "Anti-Sway Cable System for Straddle Hoists" and assigned to the assignee of the present invention, discloses an improved anti-sway system employing variable brake means incorporated into the reeving system of the hoisting cables. This system is entirely satisfactory for preventing transverse sway (generally parallel to the longitudinal axis of the horizontal gantry beam), but is less desirable for preventing longitudinal sway (generally perpendicular to the longitudinal axis of the lifting beam) in most movable gantry cranes. Such longitudinal sway may, however, be simply and effectively prevented by utilization of the apparatus of the present invention.